1. Field of the Invention
The present invention relates to a protective film for a solar cell that can prevent degradation of efficiency of a solar cell module due to oxygen and moisture penetration, can prevent degradation of performance of the protective film caused by UV rays, and thus can significantly improve efficiency and lifespan of the solar cell module, and a solar cell including the same.
2. Discussion of Related Art
Recently, solar photovoltaic power generation using solar cells has attracted attention as a next-generation energy industry. Particularly, since the energy source is clean and does not generate carbon dioxide generated when coal or petroleum is used, it can help to prevent global warming and is useful as an eco-friendly alternative energy source.
Generally, a solar cell is manufactured from a semiconductor material exhibiting the photoelectric effect, whereby electron emission is induced by incident light. When light is incident on the semiconductor material, negatively charged electrons and positively charged holes are generated, and the electrons move to a negative electrode and the holes move to a positive electrode because of a difference in potentials or concentration of charges. The solar cell is a diode generating electric power using the electrons and the holes that are concentrated on the negative and positive electrodes, respectively.
Recently, solar cells are usually manufactured by suitably combining monocrystalline silicon, polycrystalline silicon, and amorphous silicon thin films, so that thinner and more efficient solar cells can be developed and produced.
Solar cells are mounted externally, specifically, on outer walls or roofs of buildings directly exposed to sunlight, to maximize efficiency. As a protective film for protecting a solar cell module when the solar cells are exposed to an external environment for a long time, a glass substrate having various advantages such as a low coefficient of linear expansion, an excellent gas barrier property, a high degree of light penetration, a high surface planarity, and excellent thermal and chemical resistances is conventionally used.
However, the glass substrate is easily broken due to a low shock resistance, and is heavy due to a high density. For these reasons, research aimed at replacing the glass substrate with a plastic substrate is progressing.
When the glass substrate used as the protective film for a solar cell is replaced with the plastic substrate, a total weight of the solar cell module may be reduced and flexibility in design of the solar cell module may be provided. Moreover, compared with the glass substrate, the plastic substrate is strong against shock and may be economical when manufactured in a continuous process.
Meanwhile, as the protective film for a solar cell, the plastic substrate needs to have oxygen and vapor barrier properties to prevent aging of the solar cell module, UV stability, a low coefficient of linear expansion and dimensional stability to prevent distortion of the substrate according to change in the processing temperature, a high mechanical strength that is compatible with a processing apparatus used for the conventional glass substrate, a chemical resistance capable of tolerating an etching process, a high degree of light transmittance and a low double refractive index, and surface scratch resistance, but especially, oxygen and moisture barrier properties and UV stability.
However, since there is no high-functional polymer substrate film (including polymer films and polymer-inorganic material composite films) satisfying these requirements, attempts have been made to provide the above-mentioned physical properties by performing multi-layered functional coating on the polymer substrate film.